Power cascade electrophotographic development

ABSTRACT

A power cascade electrophotographic development apparatus is disclosed for applying electroscopic developer to a latent electrostatic image. The developer has a movable, flexible belt having a roughened surface which is spaced in a parallel relationship to a portion of a photoconductive drum surface to define a development zone. During development, the belt is moved upwardly thereby creating additional turbulence in the development zone beyond that normally present in cascade development.

United States Patent Genthe et al.

[ Nov. 25, 1975 POWER CASCADE ELECTROPHOTOGRAPl-IIC DEVELOPMENT Inventors: James E. Genthe, Chelmsford;

Edward F. Mayer, Acton, both of Mass.

Assignee: Itek Corporation, Lexington, Mass.

Filed: July 3, 1972 Appl. No.: 268,309

US. Cl. 118/637; 355/3 DD GOSG 13/08 Field of Search 118/636, 637; 117/17.5;

355/3 D;101/DIG. 13; 346/74 ES References Cited UNITED STATES PATENTS 8/1966 Games 118/637 X 11/1966 Lehmann... 118/637 X 4/1968 Weller 118/637 6/1971 Turner 118/637 3,662,711 5/1972 Hudson 118/637 3,678,895 7/1972 Ohta 118/637 FOREIGN PATENTS OR APPLICATIONS 889,202 2/1962 United Kingdom 118/637 Primary Examiner-Morris Kaplan Attorney, Agent, or Firm-Homer 0. Blair; Robert L. Nathans; David E. Brook ABSTRACT A power cascade electrophotographic development apparatus is disclosed for applying electroscopic developer to a latent electrostatic image. The developer has a movable, flexible belt having a roughened surface which is spaced in a parallel relationship to a portion of a photoconductive drum surface to define a development zone. During development, the belt is moved upwardly thereby creating additional turbulence in the development zone beyond that normally present in cascade development.

17 Claims, 4 Drawing Figures U.S. P at6nt Nov. 25, 1975 Sheet10f2 3,921,578

MOTOR US. Patent Nov. 25, 1975 Sheet20f2 3,921,578

BACKGROUND OF THE INVENTION l. Field of the Invention I This invention relates to electrophotographic apparatus and more particularly to an improved electrophotographic development apparatus for applying solid developer materials to latent electrostatic charge images on the surface of an insulating layer.

2. Description of the Prior Art In customary electrophotographic processes, a conductive backing having a photoconductive insulating layer thereon is imaged by first uniformly electrostatically charging its surface, and subsequently exposing the charged surface to a pattern of activating electromagnetic radiation, such as light. The radiation pattern selectively dissipates electrostatic charges in the illuminated areas on the photoconductive surface, which results in a latent electrostatic image in the nonilluminated areas. This latent electrostatic image can then be developed to form a visible image by depositing finely divided, oppositely charged electroscopic marking particles, commonly called toner, on the surface of the photoconductive insulating ,layer. Developed toner images can be fused to the insulating surface, or transferred and fused to another substrate such as plain paper.

Solid electrophotographic developer materials are customarily two component systems containing finely divided toner particles and relatively coarser, larger carrier beads. Toner particles and carrier beads are chosen or modified to make them triboelectrically dissimilar, so that triboelectric charges are generated upon each during the development process. Additionally, the components are chosen so that the toner will have triboelectric charges thereon opposite in sign to the electrostatic charge pattern to be developed. Thus, toner particles adhere to the carrier particles because of their opposite electrostatic charges and because of van der Waals forces, until they are mechanically jarred loose and captured by the opposite and stronger electrostatic charges on the insulating surface to be developed.

One type of development apparatus described in the prior art uses a belt to pull magnetic developer material by a photoconductive drum with an electrostatic image thereon. See Byrne, U.S. Pat. No. 2,832,31 1, FIG. 3. This is one type of magnetic brush development. Other general types for solid developer materials include fur brush development, impression development and powder-cloud development.

The most widely used method of developing latent electrostatic images with these two-component developer materials, however, is known as cascade" development. In normal cascade development, the developer is caused to flow by gravity over the electrostatic charge bearing surface to effect image development. As toner laden carrier beads flow over and collide with the photoconductive surface, toner is stripped from carrier beads and deposited upon the plate surface in the pattern of the latent electrostatic image thereon.

In conventional downhill cascade development, both the image bearing photoconductive surface and the flow of two-component developer material move in the same direction. Carrier beads initially give up toner material to develop the latent electrostatic image, and

then, because they have become electrostatically unbalanced, perform a second, important, function which is the cleaning of unwanted toner from background or non-imaged areas on the plate surface-a process often referred to as scavenging.

Although downhill cascade development has, in general, been found to be acceptable, there are, nevertheless, many significant problems and/or limitations which have not previously been overcome. Many modifications of the basic downhill cascade development technique have been tried to overcome such disadvantages, and some of those are as follows.

Electrically-biased development electrodes formed from metallic sheets held in a stationary position close to and parallel to the surface of xerographic plates and electrically coupled to the metal backing of the plate have been used in many cascade developers. The effect of development electrodes is to change the field configuration of the electrostatic image and to increase the field in the space above large solid areas of charge. Thus, the reproduction of high quality continuous-tone and large, solid black areas can be significantly improved. On the other hand, development electrodes cause other problems, such as blocking. This is a gradual buildup of developer on the electrode surface. Its effect is to reduce the efficiency of the electrode, and if present to a serious degree, it can even block the passage of further developer through the development zone. In extreme cases, blocking can also cause developer materials to score the delicate photoconductor surface.

Another attempt to improve cascade development is taught in Lyles, U.S. Pat. No. 3,611,992, which describes cascade development apparatus designed to rotate the xerographic drum in a direction opposite to the cascading developer. This type of development is often referred to as uphill cascade development. This method has had only marginal success, however, since the surface of the photoconductive drum is very smooth and doesnt tend to significantly modify the gravitational forces acting on developer, and since developer tends to fly away from the drum after a brief entry zone, anyway, so that the drum doesnt contact the developer through most of its fall back to a reser- VOlI'.

Although there has been a great amount of research aimed at improving traditional cascade development, significant problems still remain.

The most serious of these is the sole reliance on gravity. Because of such reliance, the points at which the developer material can be applied to and removed from xerographic drums has been severely limited, i.e., it must be applied near the top of the drum. Methods to acheive longer contact time between the electrostatic image and the developer have also heretofore been limited in cascade development. Most important, however, is the lack of control possible over significant parameters of the cascade development process when gravity alone controls flow of developer particles.

Recent research into cascade development has shown that there are certain controlling parameters. These can be broken down into three basic groups: (l) developer characteristics such as toner and carrier size and size distributions, triboelectric charge relationships, adhesive forces between toner and carrier, toner concentration, etc.; (2) dynamic parameters such as carrier density, carrier tangential velocity, carrier radical velocity, carrier angular velocity, etc.; and (3) conflgurational parameters such as development time, development zone length, bias potentials, etc. In traditional cascade development, it is possible to achieve good control over most of the developer characteristics. Although some control over the configurational and dynamic parameters can be achieved in normal cascade development, this is limited to control away from the latent electrostatic image. Nevertheless, the probability of a developing event, i.e., the capture of a toner particle by the latent electrostatic image, is strongly dependent on the capability to control the configurational and dynamic parameters of developer at the electrostatic image. To fully control the developing process, therefore, it is essential to have some control over these parameters at the latent electrostatic image.

SUMMARY OF THE INVENTION The preferred embodiment of this invention may be referred to as a power cascade xerographic development apparatus. It includes a moving belt which is positioned closely to and parallel to a portion of the photoconductive insulating surface, which is often a vitreous selenium drum. A development zone is thus defined between the belt and insulating surface. Electroscopic developer containing carrier beads and toner particles is introduced at the upper or inlet end of this zone and, after" development has taken place, developer exits from an outlet end. The belt is flexible in its longitudinal direction so that it will conform to the shape of the photoconductive insulating surface, but is preferably at least semirigid in its transverse direction so that it remains equally spaced from the surface across its width.

lt can be appreciated that this power cascade developer provides the designer of a copying machine with a new freedom of latitude. Limits previously existing on cascade development systems, including the points at 'which the developer could be introduced and the location of the apparatus itself, have been obviated or widened since developer flow is only partially dependent upon gravitational forces. For example, the developer no longer has to be introduced at the top of a xerozone can be increased. In addition, the effective length of the development zone can be significantly increased because of the increased turbulence. All of these factors allow the development of xerographic images to proceed at greater speeds thereby increasing the number of copies per minute, or allow equivalent development speeds on smaller xerographic drums which can result in lighter, more compact copy machines.

Most importantly control of the dynamic parameters can be achieved at the.latent electrostatic image be cause developer flows between two moving surfaces, i.e., the moving belt and the photoconductive, insulating surface. Because of the moving boundaries surrounding the development zone, the turbulence increases markedly over that caused solely by gravity flow of developer. Increased turbulence in the development zone causes better carrier-toner separation resulting in more activated toner being available near the photoconductor surface for development to increase development efficiency. Activated toner is created by the mechanical jarring which occurs in the development zone which helps the latent image overcome the electrostatic and va der Waals forces binding toner particles to carrier beads. Increased turbulence also provides more uniform development over the entire development zone including the elimination of direction effects often noticeable with prior cascade developers.

Increased turbulence is achieved with the development apparatus described herein by varying the belt speed and direction to modify all of the velocity vectors of developer passing through the development zone. For a fixed developer flow rate, the mean carrier tangential velocity can be reduced in the zone by moving the belt in a reverse direction thereby increasing the means carrier density in the zone; similarly, the mean carrier density can be lowered by running the belt in a forward direction to increase the means carrier tangential velocity. This means that carrier density, i.e., the number of carrier beads per unit volume of development zone, becomes controllable by adjusting the belt speed and direction. Further, the means carrier bead tangential velocity, normally dependent entirely on gravity, can be adjusted by varying the belt speed and direction. In fact, the belt motion affects all components of carrier velocity.

Radial and angular velocities are increased because of reflection of carriers from the belt. This is particularly noticeable when the belt is moved in the reverse direction.

In fact, it has been found to be particularly advantageous if the belt is driven in a direction opposite to the gravitational forces acting on the carrier beads, i.e, the reverse" direction. In this mode of operation, the tangential velocity of carrier can be lowered, thereby overcoming its natural tendency to accelerate and fly away from the drum surface. This, of course, results in better development and better scavenging because carrier and toner stay close to the photoconductor for a much longer effective time throughout the development zone. Since carrier density can be increased, a corresponding reduction in mass flow rate is possible which results in reduced abrasion of the drum surface and toner, and also provides for quieter machine operation and longer developer life. Further, the turbulence of carrier and toner is especially increased in this mode since the reverse direction of the belt particularly imparts perpendicular and angular velocity components thereto.

Blocking is also eliminated or substantially reduced with the power cascade development apparatus of this invention. This is because: (1) there is much more turbulence in the development zone; and, (2) both walls of the development zone are moving.

The net result of the new control over cascade development parameters in the development zone provided by the power cascade developer described herein is quieter machine operation, improved reliability, higher quality development, and faster development speeds or in the alternative, equivalent development speeds with smaller more compact photoconductive drums. The machine can be quieter since lower mass flow rates of developer can be used and is more reliable since problems such as blocking are eliminated. High quality is provided because of the increased turbulence which results in higher optical densities in image areas, better scavenging in background areas, and also eliminates direction effects. Finally, higher development speeds. or alternatively smaller drums can be used at equivalent speeds, because the increased turbulence increases the probability of a development event occuring, because the developer is held close to the photoconductor through essentially the entire development zone, and because the development zone can be actually or effectively lengthened.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic representation of a typical xerographic photocopying apparatus employing cascade development;

FIG. 2 is a cut-away, schematic view of one embodiment of a development apparatus of this invention;

FIG. 3 is a partially cut-away, schematic illustration of an alternate embodiment of a developer apparatus of this invention;

FIG. 4 is a partially cut-away perspective view of a section of movable belt used in the development apparatus of this invention.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION Referring now to the FIGURES in more detail, FIG. 1 illustrates a typical prior art, xerographic copy machine employing a cascade development apparatus. A xerographic drum 1 comprising a conductive metal substrate such as aluminum is covered on its outer surface with a layer of photoconductive insulating material, typically vitreous selenium. Drum 1 rotates about its axis, and is shown rotating in the same direction as the gravitational forces on the carrier-toner, i.e., the downhill direction. A uniform electrostatic charge is formed on the surface of drum 1 by means of corona charging unit 3. Uniformly charged drum 1 then passes an imaging station 5 which exposes the charged photoconductor to a light pattern representative of the original to be reproduced thereby discharging portions of the charged photoconductive surface which are struck by light. Other means of forming electrostatic latent images on insulating surfaces are known in the art and may be used instead of the one shown. More specific details on suitable photoconductive elements, charging means and exposing means can be found in the following US. Pat. Nos. 3,277,957; 3,312,548; 3,552,848; 3,566,108; 3,598,991; 3,612,864; 3,615,128; 3,628,786, and others.

Development of the electrostatic latent image is effected by a traditional cascade development apparatus 6, which includes a number of elements surrounded by housing 7. A conveyer system is formed by a belt trained to pass over two drive rollers 11 and 12. Belt 10 has buckets 13 attached thereto. Electroscopic developer is lifted from a reservoir 15 in buckets 13 to a point at the upper portion of drum 1 where it is dispensed onto feed guide 17. As developer cascades over drum 1, toner particles separate from carrier beads and deposit on the drum surface in accordance with the latent electrostatic latent image thereon, thus forrning a visible toner image. Spent developer is guided back into reservoir 15 by guide 19. A biased development electrode 21 is shown closely spaced to the drum surface to improve solid area coverage.

As can be seen, cascade development of this type depends largely upon gravitational forces for the flow of developer across the electrostatic latent image, and

very little control over the dynamic parameters can be achieved. The only moving surface in the development zone is the photoconductive drum, and these usually have smooth surfaces which tend to disturb the developer flow due to gravity very little. Additionally, since so much reliance on gravity is required, the choice of positioning the developer apparatus is severely limited.

After the electrostatic image has been developed, the toner image is transferred from the surface of drum 1 to the copy medium, which is usually paper. Toner transfer and fusing station 25 typically includes a web of paper, dispensing, windup and guide rolls for the web, a corona charging unit, and a heater. More specific details of these elements can be found in US. Pat. Nos. 3,584,195; 3,642,362; 3,647,292, and others. After the toner transfer and fusing station, a cleaning brush 27 removes residual toner from the surface of drum 1.

The charging, exposure, transfer, fusing and cleaning techniques shown in FIG. 1 are believed to be representative of those commonly used. Nevertheless, many other systems may be employed in connection with the power cascade xerographic development apparatus as described herein.

FIG. 2 is a schematic sectional view of one embodiment of the power cascade xerographic developer of this invention. A housing 30 surrounds the various elements and helps to contain any powder dust generated therein. Xerographic developer material 32 is raised from the reservoir portion 34 of the development apparatus to an elevated position by bucket elevator 36 comprising a belt 38 having attached thereto buckets 40. Belt 38 is trained to pass over drive roller 42, which is driven by motor 43, and guide roller 44. At the top of the bucket elevator 36, developer material falls from buckets 40 over adjustable guide baffle 46 into feed hopper 48. Developer material is dispensed from hopper 48 by orifice 50, which can be adjusted to control the mass flow rate.

Developer material dispensed from hopper 48 enters a development zone formed between the surface of xerographic drum 52 and movable belt 54. Belt 54 is an endless loop trained over drive roller 56, driven by motor 57, and guide rollers 58 and 60 so that the run of the belt consists of three distinct portions. In the first portion, extending between rollers 56 and 58, the belt travels in a closely spaced and parallel relationship to drum 52, but in the opposite direction. After the belt passes around rollers 58, it travels away from the drum 52 to roller 60, and then starts its third path back to roller 56. Developer exiting from the development zone formed between drum surface 52 and movable belt 54 is directed by semicircular baffle 69 back to reservoir 34.

Movable belt 54 is constructed of a flexible material such as a silicone rubber. In the embodiment shown, belt 54 has stiffeners 62 extending through the belt in a transverse direction to make the belt semirigid in the width direction, which is preferred. The stiffeners help in maintaining the belt an equal distance from drum 52 across its width. Also, the ends of stiffening rods 62 can be inserted in a drive ring mounted at the end of the drum to provide a convenient drive for the belt, if desired.

In order to maintain a uniform mass flow rate of developer from hopper 48, the orifice 50 is adjusted to a predetermined opening and developer is allowed to fill hopper 48 to the point where it overflows. The overflow developer is caught in a return chute formed by elements 61 and 62. Baffles 64 reduce the velocity of returning overflow developer to eliminate unnecessary powder cloud generation in the reservoir 34.

Disc 68 can be moved toward or away from the downhill path of belt 38 to thereby act as a tensioning device for belt 38. Baffle 66 helps to furtherseal off the reservoir portion 34 of developer apparatus 36, helping to contain any loose toner particles therein.

A significant feature of this inventionis that the belt moves during development. The belt has a much greater effect on developer than the photoconductive surface. This is because the belt can have a rough surface whereas the photoconductor is smooth, and also because the developer tends to fly away from the photoconductive drums surface due to centrifugal force. In regard to the latter, the belt acts to contain developer and to redirect developer falling away from the drum back towards the drum surface. If run in the reverse direction, it also lowers the mean carrier tangential velocity.

Increased turbulence results from the movable member or belt causing substantial changes in the magnitude and direction of the velocity vectors of carrier beads descending through the development zone under the influence of gravity. Carrier beads are forced to move randomly rather than in a pattern wherein nearly all of them have velocity vectors of substantially the same magnitude and direction. The increased turbulence provides a greater number of carrier collisions per unit volume of development zone thereby shaking loose more toner particles and increasing the probability of a development event occurring.

The belt can be moved in a forward or reverse direction, regardless of whether the photoconductive drum is held stationary, or moved in an uphill or downhill direction. For purposes of this description, forward means the same direction as the gravitational forces on the developer and reverse means the direction opposite to the gravitational forces. In either the forward or reverse embodiment, the belt should have a linear velocity of at least one inch per second in order to have a significant effect. Preferably, the belt has a forward or reverse linear velocity of from about to 40 inches per second. In most cases, of course, the drum will be moving, and the most preferred case is to run the belt within the preferred ranges above and also from about 0.5 to about 4 times the linear velocity of the drum surface for both the forward and reverse embodiments and for both uphill and downhill drum direction.

The movable member or belt is preferably spaced a distance equal to between about three and ten carrier diameters from the photoconductive surface. For most of the commonly available xerographic developers, this distance is between about 0.030 and 0.150 inches.

FIG. 3 is a cross-sectional schematic view of an alternate developer apparatus of this invention. As shown, this developer apparatus is used with a drum 71 rotated about a center shaft 72 in a downhill direction. A sheet of zinc oxide paper 73 having an electrostatic image thereon is shown about to enter the development zone of development apparatus 74. Paper 73 can be fastened to drum 71 any conventional means such as releasable jaws.

Development apparatus 74 has an outer housing 75 which can be, for example, sheet metal. A belt 76 is shown and is similar to that described above in FIG. 2 and as further described in FIG. 4 below. Belt 76 passes around drive roller 77, driven by motor 90, and guide rollers 78 and 79, and travels in the same direction as the surface of drum 71. Spacers 80 help to maintain belt 76 an equal distance from guide baffle 81.

Development apparatus 74 also contains an elevator belt 82 trained to pass in a vertical direction over guide roller 83 and roller 84, which is driven by motor 91, and to work in cooperation with belt 76 to form an elevator for developer material 85 stored in reservoir section 86. Spacers 87 are attached to belt 82 to maintain it the desired distance from developer belt 76 in the vertical elevator portion of the apparatus. A guide 88 is placed behind the other side of elevator belt 82 to further help in maintaining proper spacing in the elevator section thereof. Developer 85 collects at the top of the elevator section and enters inlet section 89 of the development zone formed between drum 71 and belt 76. Because belt 76 moves developer along, it is possible to extend the development zone further than would be possible with a normal cascade development system which relies solely on gravity for developer transport. As can be appreciated from FIG. 3, developer belt 76 assists gravitational forces in the downhill verticle sections of the development zone, and substitutes for them in horizontal sections or uphill verticle sections.

FIG. 4 is a partially cutaway prospective view of a belt suitable for use in the development apparatus of this invention. The belt 100 can be a silicon rubber or other flexible material such as polyurethane or even metal. Preferably, of course, the belt material will be triboelectrically compatible with the developer. Belt 100 has a relatively thin surface with saw tooth backing members 103, which provides it with strength but flexibility in the longitudinal direction. Cylindrical rods 102 extend through reinforcing members 103 across the width of belt 100 to stiffen it in the transverse direction. In the embodiment shown, the belt itself is nonconductive with conductive areas 104 regularly spaced thereon. Also, the entire belt can be conductive. In either case, the belt can serve the function of a development electrode, i.e., to enhance solid area coverage or alternately to suppress background.

Roughening the belt surface increases the effect of the belt. Knurled surfaces, ridge patterns, knap surfaces, etc. can be used. Roughened surfaces are particularly preferred where the belt velocity is low.

A power cascade electrophotographic development apparatus similar to the one illustrated in FIG. 2 was used to produce actual experimental data. An 8-inch diameter aluminum drum with a zinc oxide Electrofax sheet pinned thereto was used as the photoconductor. A cast silicone rubber belt with approximately 0.150 inch diameter steel reinforcing rods spaced uniformly inches apart and extending through the belt was used. This belt is similar to that illustrated in FIG. 4, except that it was totally dielectric. The width of the belt was 9 inches, which provided and 8 /2 inch development area on the sheet. A development zone was formed with of arc wherein the top of the zone was positioned at 60 from the horizontal so that the zone consisted of both normal and inverted cascade development. The spacing between the belt and photoconductor surface was maintained at a uniform distance of 0.080 inches. I-Iunt Chemical 50-25 negative-working electroscopic developer was introduced in the developmentzone at a mass flow rate of about 30 grams per inch per second. The Electrofax sheets were uniformly negatively charged to a surface potential of about 400 volts and contact printing methods were used to establish a latent electrostatic image of line copy thereon.

In one set of experiments, designed to isolate the effect of the belt, the charged and exposed zinc oxide sheets were held stationary in the development zone and developer was poured through the zone for two seconds. When the belt and drum were both held stationary, strong development was found only over an arc of some 20 at the top of the zone. This would be expected with a conventional free fall cascade developer because of the horizontal momentum acquired by the carrier causing it to follow a parabolic trajectory that departs from the photoconductor surface. Very weak irregular development associated with irregularities in the belt surface was found in the rest of the development arc. When the experiment was conducted with the belt running forward at a speed of 20 inches per second, similar strong development was found at the top of the zone together with weak but uniform development over the rest of the zone indicating a tendency towards laminar flow with centrifugal forces causing the carrier to reside mainly on the belt surface. When the experiment was conducted with the belt running in reverse at 20 inches per second, strong development was found over the entire 90 arc of the development zone, the top of 20 being marginally stronger. These experiments demonstrate that both belt directions improve development, and that in the reverse mode the belt drastically modified the carrier density and velocities over the greater part of the zone, thus increasing the frequency of developing events.

Another set of experiments was performed with the same equipment wherein the photoconductor was rotated through the development zone at a linear speed of 15 inches per second. The belt was run at a speed of 30 inches per second in both the forward and reverse directions. The reflection optical density in character areas was measured. In the forward mode, an optical density of 0.9 was obtained, corresponding to a development time of 0.4 seconds. In the reverse mode, an optical density of 1.4 was obtained.

Other experiments were run varying the photoconductor speed and keeping the belt running in the reverse mode at two times the photoconductor speed. Curves of the reflection optical density in character areas versus development time indicated that the reverse mode is progressively more advantageous as development time is decreased.

A further set of experiments was run on a similar apparatus, except that the drum size was reduced to inches. To compensate for the smaller drum, the development zone was extended to an arc of 120. When the drum was rotated at inches per second and the belt was run in the reverse direction at inches per second, optical densities were obtained which appeared equivalent to those obtained with the larger drum.

What is claimed is:

l. A cascade development apparatus for developing an electrostatic latent image on the surface of an insulating layer with electroscopic developer containing a mixture of carrier beads and toner particles, comprising:

a. a movable member having a roughened surface positioned adjacent to at least a portion of the surface of said electrostatic latent image bearing layer to define therebetween an elongated development zone having an inlet and outlet for developer material, the development zone having a spacing be- 5 tween said roughened surface and said surface sufficient to allow developer material to pass therethrough, said spacing being between about 3 and about 10 carrier bead diameters;

b. means for driving said movable member upwardly with a linear velocity of at least five inches per second thereby creating between said member and said insulating surface increased turbulence in the development zone as developer material passes therethrough; and,

c. means for introducing developer material at the inlet of said development zone.

2. An apparatus of claim 1 wherein said movable member is positioned in a closely spaced, substantially parallel relationship to at least a portion of the surface of the electrostatic latent image bearing layer.

3. An apparatus of claim 2 wherein said movable member comprises a belt.

4. An apparatus of claim 3 wherein said means for driving causes said belt to move in a reverse direction with respect to direction of movement imparted to said insulating layer, and means to impart said movement.

5. An apparatus of claim 4 wherein said insulating surface has the shape of a drum and comprises a photoconductive insulating material bonded to a conductive substrate.

6. An apparatus of claim 5 wherein said belt is fabricated from silicone rubber.

7. An apparatus of claim 6 wherein said belt has transverse stiffening rods therethrough to stiffen it in its transverse direction thereby helping to maintain an equal spacing from said photoconductive insulating surface.

8. An apparatus of claim 7 wherein said photoconductive insulating drum is moved in a downhill direction.

9. An apparatus of claim 8 additionally containing reservoir means for storing electroscopic developer.

10. An apparatus of claim 9 additionally containing means for transporting electroscopic developer from said reservoir means to the inlet of said development zone.

11. An apparatus for developing with electroscopic developer an electrostatic latent image on the surface of a rotatable drum, comprising:

a. a movable belt member having a roughened surface positioned relatively close to and substantially parallel to at least a portion of said surface, the belt member and drum surface thereby defining a development zone for electroscopic developer to flow through by gravity;

b. means to drive said movable belt member upwardly with a linear velocity in the reverse direction with respect to said drum of at least five inches per second to increase the turbulence of developer falling through said development zone; and,

c. means to introduce electroscopic developer at the upper end of said belt member so that it flows through said development zone.

12. An apparatus of claim 11 wherein said belt is spaced from said drum surface a distance of from about three to about ten carrier diameters.

16. An apparatus of claim 15 further including means for collecting developer material exiting from development zone and means to transport developer material from said means for collecting to said means for introducing.

17. An apparatus of claim 11 wherein said development Zone is elongated. 

1. A cascade development apparatus for developing an electrostatic latent image on the surface of an insulating layer with electroscopic developer containing a mixture of carrier beads and toner particles, comprising: a. a movable member having a roughened surface positioned adjacent to at least a portion of the surface of said electrostatic latent image bearing layer to define therebetween an elongated development zone having an inlet and outlet for developer material, the development zone having a spacing between said roughened surface and said surface sufficient to allow developer material to pass therethrough, said spacing being between about 3 and about 10 carrier bead diameters; b. means for driving said movable member upwardly with a linear velocity of at least five inches per second thereby creating between said member and said insulating surface increased turbulence in the development zone as developer material passes therethrough; and, c. means for introducing developer material at the inlet of said development zone.
 2. An apparatus of claim 1 wherein said movable member is positioned in a closely spaced, substantially parallel relationship to at least a portion of the surface of the electrostatic latent image bearing layer.
 3. An apparatus of claim 2 wherein said movable member comprises a belt.
 4. An apparatus of claim 3 wherein said means for driving causes said belt to move in a reverse direction with respect to direction of movement imparted to said insulating layer, and means to impart said movement.
 5. An apparatus of claim 4 wherein said insulating surface has the shape of a drum and comprises a photoconductive insulating material bonded to a conductive substrate.
 6. An apparatus of claim 5 wherein said belt is fabricated from silicone rubber.
 7. An apparatus of claim 6 wherein said belt has transverse stiffening rods therethrough to stiffen it in its transverse direction thereby helping to maintain an equal spacing from said photoconductive insulating surface.
 8. An apparatus of claim 7 wherein said photoconductive insulating drum is moved in a downhill direction.
 9. An apparatus of claim 8 additionally containing reservoir means for storing electroscopic developer.
 10. An apparatus of claim 9 additionally containing means for transporting electroscopic developer from said reservoir means to the inlet of said development zone.
 11. An apparatus for developing with electroscopic developer an electrostatic latent image on the surface of a rotatable drum, comprising: a. a movable belt member having a roughened surface positioned relatively close to and substantially parallel to at least a portion of said surface, the belt member and drum surface thereby defining a development zone for electroscopic developer to flow through by gravity; b. means to drive said movable belt member upwardly with a linear velocity in the reverse direction with respect to said drum of at least five inches per second to increase the turbulence of developer falling through said development zone; and, c. means to introduce electroscopic developer at the upper end of said belt member so that it flows through said development zone.
 12. An apparatus of claim 11 wherein said belt is spaced from said drum surface a distance of from about three to about ten carrier diameters.
 13. An apparatus of claim 12 wherein said latent electrostatic image is contained on the surface of an insulating sheet fastened to said drum.
 14. An apparatus of claim 12 wherein said drum comprises a photoconductive insulating material bonded to a conductive substrate.
 15. An apparatus of claim 14 wherein said drum surface comprises vitreous selenium.
 16. An apparatus of claim 15 further including means for collecting developer material exiting from development zone and means to transport developer material from said means for collecting to said means for introducing.
 17. An appAratus of claim 11 wherein said development zone is elongated. 